For measuring direct current and alternating current, the compensation principle is used frequently, in which the magnetic field produced by a measurement current in a magnetic core is compensated by a compensation current in a secondary winding. For controlling the compensation current, a detector is provided in the magnetic circuit, which detector detects the deviation from the zero field. The secondary current is thus a precise image of the primary current to be measured. Compensation current sensors of such type are known by way of example from EP 0294590 B1 or EP 0960342 B1. The advantage of such current sensors is that they have a very high degree of accuracy and practically do not require any intervention in the electric circuit to be measured. On the other hand, the disadvantage is that compensation current sensors require relatively high expenditure.
Tests for battery management modules, which are mounted on the battery pole terminal and which are supposed to have a digital output for connection to data buses (e.g., LIN bus or CAN bus), have shown that the micro controller required in this case, with the analog/digital transducer, reference voltage and the high-precision load resistors represents a formidable expense factor.
However, in order to achieve the required precision, a micro controller would also be necessary in a solution with an analog output (e.g., voltage output) in the low current region particularly for hysteresis determination. In addition, an analog output is interference-prone and a precision reference is required on both the sides so that the output signal regions of the sensor and the analog/digital transducer are attuned to each other in the control unit processing these further.
Sensors having a pulse width-modulated output (PWM output) are also used. Here, a pulse width-modulated output signal (PWM output signal) is generated by converting a linear output signal into a pulse width-modulated signal (PWM signal). A PWM signal is very advantageous per se since it is fail-safe and can be evaluated very easily and cost-effectively in a controller of a control unit processing it further, there being particularly no requirement of any cost-intensive analog-digital converters and precision references. However, the approach for generating a pulse width-modulated output signal from a linear output signal by a corresponding conversion, is disadvantageous in many respects. Considerable additional costs are involved since an additional step, in which precision components are also used, is required for converting the linear output signal into a pulse width-modulated signal. Furthermore, said solution is also problematic in terms of reliability and security. Thus, for example, a fault is feasible in which the pulse width-modulated signal with a duty factor of 1:1 indicates a normal operating state and thus does not indicate any primary current although the sensor arranged in front has failed and there is a possibility of a very high primary current flowing.